1. Field of the Invention
The invention relates to an adjustable-length compression spring having a casing filled with pressure fluid; a guide and seal unit which closes the casing at a first end; a piston rod which, through the guide and seal unit, is extended from, and sealed towards, the first end and has an outer end; a piston which is joined to the piston rod and sealingly guided in the casing; a first sectional casing chamber which is unilaterally defined by the piston and filled with pressure fluid; a second sectional casing chamber which is connectable to the first sectional casing chamber and filled with pressure fluid; an operating valve for connection to each other of the sectional casing chambers, the valve comprising a valve pin to be actuated from outside the casing by an operating element, the valve pin being displaceable between a position of release and a position of blocking such that in the position of release of the valve pin, the connection between the sectional casing chambers is released; and in the position of blocking of the valve pin, the connection between the sectional casing chambers is shut off.
2. Background Art
Compression springs of the generic type are known from DE 36 02 441 A1, DE 42 35 435 A1 as well as from prior public use, having a field of application for instance in office chair, in which case they are used for height adjustment of seats as well as inclination adjustment of seating areas and backrests for example in known synchronous mechanisms. This is intended to create the possibility of permanently keeping the compression spring in a position of release, providing for example for seesaw motion of the synchronous mechanism. Quite a few solutions have been suggested for durably keeping the compression spring in a position of release, among which figure lockable lever mechanisms of comparatively complicated structure. EP 0 052 832 A2 describes a locking-button release of a gas spring included in a control head that will be screwed on to the free end of the piston rod of the gas spring. Although this control-head type solution has been available for more than twenty years, it has never proved successful apparently due to constructional problems posed in particular by adaptation of the control head to a respective gas spring and the dimensional requirements involved.